It has been the common practice to mount variable pitch propulsor blades, such as propeller blades or fan blades of aircraft engines, to a central rotating hub by retaining each blade root in its own retention socket, a plurality of which are circumferentially spaced about and extend radially outwardly from the central hub. Customarily, each blade root is retained within its retention socket for rotation therein about a pitch change axis by means of an inboard angular contact ball bearing assembly and a circumferential outboard bearing assembly, each operatively disposed intermediate the blade root and the inner wall of the retention socket. For example, in the retention system described in commonly assigned U.S. Pat. No. 3,490,537, both the inboard angular contact bearing and the outboard circumferential bearing are ball bearings, and both are disposed and supported in part in a bearing race formed in the shank of the blade root and in part in a cooperating bearing race formed in the inner wall of the retention hub. In U.S. Pat. No. 4,921,403, however, the outboard circumferential bearing is a roller bearing rather than a ball bearing, and both the inboard angular contact ball bearing and the outboard circumferential roller bearing are disposed and supported in separate bearing races which are seated in the blade root and the inner wall of the retention socket rather than being integrally formed therein.
In these type retention systems, the outboard circumferential bearing, whether a ball bearing or roller bearing, is not preloaded and serves simply to limit bending motion. Such retention bearings have proven quite satisfactory for use with heavier prior art propulsor blades, for example steel or aluminum propeller blades and titanium fan blades, since these blades weigh enough that they experience sufficient centrifugal loading when rotating at operating RPM that rocking of the retention bearings does not occur. However, modern lightweight hollow shell and spar construction blades, such as blades wherein the blade shell and/or blade spar are made of lightweight composite materials, may not experience sufficient centrifugal loading to prevent the bearing elements on one side of the bearing from becoming unloaded under the influence of bending loads imposed upon the blade during operation. As a result, high blade cyclic bending loads during climb can lead to fretting wear and/or excessive contact stresses.
One method of preventing rocking is to simply increase the pitch diameter of the bearing. However, doing so increases the weight of the blade retention system thereby offsetting much of the desired weight reduction achieved by using lightweight composite blades. Further, increasing the pitch diameter of the bearing leads to increased blade root thickness, thus resulting in poorer aerodynamic performance and higher secondary stresses in the hollow composite blade root structure.
Another method is to augment the centrifugal loading experienced during operation by preloading the bearings to adequately counter the bending loads so as to prevent such undesirable blade rocking. For example, in U.S. Pat. No. 3,040,817, the inboard and outboard bearings are preloaded by a threaded member which is screwed into an internally threaded cavity in the blade root so as to pull the blade inboardly such that the bearings are maintained in contact with their respective races between the blade root and the encompassing retention socket. While functional, such a retention arrangement includes a threaded interface in the primary load transfer path from the blade root to the hub and, therefore, provides a discontinuity at which stress may undesirably concentrate when the blade undergoes high centrifugal loading during operation.
The retention system disclosed in commonly assigned U.S. Pat. No. 4,850,801, also utilizes a threaded member as a means of preloading the outboard anti-rocking bearing, but moves the discontinuity provided by the threaded interface to a location outside the primary load transfer path. As disclosed therein, the outboard anti-rocking bearing comprises an angular contact ball bearing which is preloaded by an annular fastener threaded into the socket so as to pull the blade outboardly thereby preloading this angular contact bearing between a bearing race on the blade root and a bearing race on the annular fastener. The annular preloading fastener also serves to support a relatively small diameter O-ring static seal and a relatively large diameter O-ring dynamic seal. Although all discontinuities due to the interengagement of the threaded preloading fastener are advantageously moved out of the primary load path to a radially outer location on the hub arm where the stresses are lower, the discontinuities at the threaded interface still exist.